An oven can be one of the most difficult kitchen appliances to clean. Food that splatters onto the interior surfaces of an oven during cooking usually becomes baked-on, making the removal thereof particularly difficult. If the oven is not frequently cleaned, the amount of baked-on food can build up rapidly, thereby increasing the cleaning difficulty.
Coatings used for the interiors of ovens are primarily one of three types: (1) self-cleaning pyrolytic ground coats, (2) non-self-cleaning ground coats, and (3) catalytic continuous clean enamels.
The first type of coatings, i.e. self-cleaning pyrolytic ground coats, enable food residue to be reduced to ash by exposure to temperatures between about 850 and 1000° F. (455 and 538° C.). However, there are several concerns associated with heating oven coatings to such temperatures. First, high temperatures are required, necessitating extra insulation around the oven chamber and safety interlocks for oven operation. Second, producing such high temperatures requires relatively large amounts of energy consumption. Third, depending upon the materials exposed to such high temperatures, concerns exist as to the possible release of toxic fumes. Fourth, the cleaning cycle used in association with these coatings takes up to three hours to complete and potentially reduces the overall service life of the oven. Further, in order to withstand multiple cleaning cycles, such enamel coatings generally contain hard, chemically-resistant frits that, without high-temperature exposure, have inherently poor release properties, thereby compounding the difficulty in removing baked-on residues.
The second type of coating, i.e. non-self-cleaning ground coats, requires significant cleaning efforts by the consumer and/or harsh alkaline saponifying cleaners that have a pH of approximately 14. As will be appreciated, significant safety concerns exist when using, handling, and storing such hazardous and often toxic cleaners. In addition, producing various oven models each with potentially different interior coatings can increase manufacturing complexity and thus costs. In order to provide a lower cost appliance, Original Equipment Manufacturers (OEMs) generally use the same pyrolytic enamel as used in self-cleaning ovens, but do not equip the oven with a self-clean cycle. Thus, although satisfactory, many ovens such as those lacking a self-cleaning cycle, utilize coatings that are not optimally designed for the oven.
The third type of coating, i.e. catalytic continuous clean enamels, fire out with a porous microstructure, enabling the reduction of food residue to ash at normal cooking temperatures. Although satisfactory in many regards, these coatings have largely fallen out of use in North America but are still in use in other markets.
The patent literature has described enamel cover coatings. U.S. Pat. No. 7,005,396 describes enamel formulas that include mixtures of low softening point alkali aluminophosphate frit with a high softening point zirconia phosphate frit. The soft frit fluxes the hard frit, creating workability within typical oven enamel firing conditions of 1520 to 1600° F. (827 to 871° C.). The fired coating sheds baked-on foodstuffs upon exposure to a moist environment. The soil release mechanism is hypothesized to be due to relatively weakly bonded surface absorbed water rather than the significantly stronger bonds otherwise occurring on conventional enamels. When applied to an oven cavity, this surface creates a fourth option for soil removal through relatively brief exposure to water or steam at much lower temperatures than used with the pyrolytic enamels and without the use of harsh alkaline cleaners.
However, the glasses discussed in U.S. Pat. No. 7,005,396 have certain characteristics that preclude their use in oven applications using ground coats or that render them difficult to apply with dry electrostatic methods to oven surfaces. The thermal expansion of the glasses is too high relative to ground coats typically used in ovens. This difference in thermal expansion characteristics would lead to cracking or other distortions in an oven coating using such glasses. In addition, the glass temperature of the glasses is below 750° F. (400° C.). On hidden bake ovens in which an oven floor covers the heating element, the floor can exceed these temperatures. Foods baked-on above the glass temperature would then not necessarily release upon exposure to moisture. As for their application characteristics, a soft and hard frit with mill-added raw materials described in that patent would tend to segregate on recirculation through an automatic spray booth and thus this characteristic detracts from application of the glass mixture via spraying.
Accordingly, there exists a need for a composition that can be applied to the interior surfaces of oven cavities and other articles from which baked-on foods can be easily removed without the need for pyrolysis or highly alkaline cleaners.